Col. Frank Milstead, director of the Public Safety Department, said he would like to thank the man who stepped in. "I don't know that my trooper would be alive today without his assistance," he said, according to CNN affiliate KNXV-TV in Phoenix.

Unusual circumstances

Public safety spokesman Bart Graves said the incident began with the trooper responding to report of shots fired from the median of an unlit, rural section of Interstate 10 near Tonopah. Andersson came across a vehicle that had rolled over and got out to investigate. While Andersson was setting flares, the suspect shot him.

It was not clear where the suspect was when the officer pulled up.

Milstead said at some point the suspect started beating Andersson, who was unable to use his right arm to defend himself. That's when the motorist pulled up and opened fire on the suspect.

"It's fairly rare" that citizens who come to the aid of law enforcement officers use deadly force, Cecil said.

Another motorist who stopped used the trooper's portable radio to call in the shooting.

The conversation between that driver and the dispatcher was very businesslike, Cecil said.